increased vacuum with increased ig timing

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GARY C
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Re: increased vacuum with increased ig timing

Post by GARY C » Sun Dec 10, 2017 3:56 pm

Belgian1979 wrote:
Sun Dec 10, 2017 3:52 pm
GARY C wrote:
Sun Dec 10, 2017 3:49 pm
Belgian1979 wrote:
Sun Dec 10, 2017 3:45 pm
OK, and how do you know when you've gone too far or are at a correct timing ? I mean, if vacuum keeps dropping this hardly looks like the factor to determine the correct ignition timing.
Dropping? you OP said increasing?
Can someone explain to me what causes the engine to pull more vacuum when ignition timing is increased ?
Sorry, map drops (expressed in kPa) but this means that vacuum increases...my fault.
Don't confuse me, I only have a few brain cells left.. :)

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Re: increased vacuum with increased ig timing

Post by Belgian1979 » Sun Dec 10, 2017 3:58 pm

Well, map is the engine manifold air pressure, but it is measured from total vacuum and up. So atmospheric pressure is ca. 101 kPa. My engine usually has around 69 kPa of manifold pressure and I've had up to around 64 kPa. In real world terms this would be in between 8 inch and 10-11 inch.

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Re: increased vacuum with increased ig timing

Post by joe 90 » Sun Dec 10, 2017 4:09 pm

A good well built well tuned street engine will idle with a vac of 18 to 20 inches HG.
That in turn is about 1/3 atmosphere or 35 ish KPa.
You should be able to get a good smooth idle at 750 ish RPM.

Otherwise you've got things to fix.
viewtopic.php?f=15&t=49366

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Re: increased vacuum with increased ig timing

Post by F-BIRD'88 » Sun Dec 10, 2017 6:57 pm

If this is on your corvette with high overlap camshaft the increased idle timing causes the slow(er) burning exhaust infected air fuel charge at a better time in the combustion event. The exhaust gas slows the combustion speed down. The advanced the timing at idle corrects the combustion timing to make more better use of the fuel burn.
The engine torque at idle increases thus more rpm and manifold vacuum with less carb or throttle body opening at idle. With the exhaust re-mix delution of the cam overlap, at idle you do not want to depend on the computer and O2 sensor AFR at idle as the AFR and cylinder to cylinder AFR will not be uniform at idle.

You'll get better idle with pre-fixed idle fuel tables based on temperature rpm and idle air control motor.

Up until the motor is operating at higher rpm. Then switch to MAP and O2 sensor output for fuel curve.

Engines with high overlap/duration racey cams like lots of ignition timing at idle to correct for the
slower burn effect of that exhaust gas in the combustion mix. Better idle quality more manifold vacuum at idle,
More,, better throttle response. It "drives" better. Exhaust gas does not burn, twice.

With the high cam overlap and exhaust re-mix at idle and low rpm the engine is much more UN likely to spark knock from lugging the motor as a stock motor with low overlap would tend to if with very agressive timing at or off idle.
The rules of a stock type engine do not apply on a racey cammed up motor. Two different beasts.
The cammed up motor really likes the extra spark advance at idle and low speed running.
The best advance curve will be very different.

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Re: increased vacuum with increased ig timing

Post by PackardV8 » Sun Dec 10, 2017 7:18 pm

Distributors and carbs aren't usually able to handle that reliably.
I disagree and have done it with carb, HEI and adjustable vac advance (actually becomes a vac retard at load when hooked to full vac port), . . . . It does require the correct spring and weight package along with an adjustable vac port and some actual work to get them in sink.
Gary, you obviously have the ability to make a carb and distributor function better than they USUALLY do. Those of us who wrestled with them back in the days when the OEMs couldn't make them run well and meet emissions were probably scarred forever.
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Re: increased vacuum with increased ig timing

Post by GARY C » Sun Dec 10, 2017 7:26 pm

PackardV8 wrote:
Sun Dec 10, 2017 7:18 pm
Distributors and carbs aren't usually able to handle that reliably.
I disagree and have done it with carb, HEI and adjustable vac advance (actually becomes a vac retard at load when hooked to full vac port), . . . . It does require the correct spring and weight package along with an adjustable vac port and some actual work to get them in sink.
Gary, you obviously have the ability to make a carb and distributor function better than they USUALLY do. Those of us who wrestled with them back in the days when the OEMs couldn't make them run well and meet emissions were probably scarred forever.
I never said anything about emissions, they were all older exempt cars. I only wish I had learned this timing curve before mid 2000 because I would have been a lot happier with all of my previous hot rods.

The car I learned it on went from about a 900 rpm idle, 10" vac to 650 idle and 16" vac, my muffler guy thought I changed cams the next time he heard it.

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Re: increased vacuum with increased ig timing

Post by Belgian1979 » Mon Dec 11, 2017 10:14 am

Also, I see it get richer when I decrease timing (along with more airflow), leaner when increasing the timing (less airflow). Factoring a closed loop idle control I can see how the amount of airflow would do this, but I'm not seeing how this could effect the air/fuel as such. In fact I would have expected the opposite.

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Re: increased vacuum with increased ig timing

Post by joe 90 » Mon Dec 11, 2017 3:17 pm

Your fuel pressure will be all wrong.....as per normal with modded EFI engines.

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Re: increased vacuum with increased ig timing

Post by Belgian1979 » Mon Dec 11, 2017 3:50 pm

joe 90 wrote:
Mon Dec 11, 2017 3:17 pm
Your fuel pressure will be all wrong.....as per normal with modded EFI engines.
Wrong and even so, when using EGO it would correct any inconsistencies in fuel pressure.

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Re: increased vacuum with increased ig timing

Post by joe 90 » Mon Dec 11, 2017 7:18 pm

Software never works on broken hardware.

So no, you can't tune for it.
Because it's an added variable with no input to the ECU.


So........what's you nominal base fuel pressure?
What does the gauge read when it's idling?
How much manifold vac at idle?
What does the gauge read at idle when you remove the vac line?
What does the gauge read when you shut the engine off?

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Re: increased vacuum with increased ig timing

Post by joe 90 » Mon Dec 11, 2017 9:34 pm


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Re: increased vacuum with increased ig timing

Post by Belgian1979 » Tue Dec 12, 2017 3:23 am

joe 90 wrote:
Mon Dec 11, 2017 7:18 pm
Software never works on broken hardware.

So no, you can't tune for it.
Because it's an added variable with no input to the ECU.


So........what's you nominal base fuel pressure?
What does the gauge read when it's idling?
How much manifold vac at idle?
What does the gauge read at idle when you remove the vac line?
What does the gauge read when you shut the engine off?
43.5 psi. So when having the engine idling the pressure reads exactly the amount of psi lower than the vacuum at idle.

If you would know your stuff, you would know that if for instance the fuel pressure would be lower, the EGO correction would detect this as a leaner than normal mixture and the ecu would automatically correct for it. If you haven't already incorporated it in the basic tune.

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Re: increased vacuum with increased ig timing

Post by joe 90 » Tue Dec 12, 2017 3:01 pm

Belgian1979 wrote:
Tue Dec 12, 2017 3:23 am




If you would know your stuff, you would know that if for instance the fuel pressure would be lower, the EGO correction would detect this as a leaner than normal mixture and the ecu would automatically correct for it. If you haven't already incorporated it in the basic tune.

It can't correct quickly (in real time) and it can only correct by a certain %.


So what PSI on the gauge at idle?
You haven't answered that ?

5 questions in total.
That's how you know if your fuel system is up to spec..........which with people who HAVE to ask lots of questions because they can't figure out anything for themselves......it's ALWAYS wrong.




If it really did what you're trying to say it does....which it doesn't...... (O2 feedback changing fuel trims to lock lambda at 1) then you're NOT going to see it get richer as you change idle speed with timing.

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Re: increased vacuum with increased ig timing

Post by naukkis79 » Sun Dec 17, 2017 4:32 am

Belgian1979 wrote:
Mon Dec 11, 2017 10:14 am
Also, I see it get richer when I decrease timing (along with more airflow), leaner when increasing the timing (less airflow). Factoring a closed loop idle control I can see how the amount of airflow would do this, but I'm not seeing how this could effect the air/fuel as such. In fact I would have expected the opposite.
Measured with lambda sensor? False readings, as other are saying you should newer use closed-loop at idle with cammed motors.

As you increase timing at idle vacuum increases as residual exhaust gases and combustion isn't complete. So unburnt oxygen makes lambda sensor to report lean mixture which isn't true at all.

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Re: increased vacuum with increased ig timing

Post by F-BIRD'88 » Tue Dec 19, 2017 9:02 am

You will have that Corvette motor wore out before you get a nice civil idle with that dirt track cam in it.

You seem to do a hell of a lot of idling a low speed driving.
Get a camshaft that reflects how you actually drive your car.
A lot LESS valve overlap. Then the efi will be nice.
Overall car use performance will be vastly improved.
Race cars make crappy street cars.

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